The present invention is directed to a double band press for applying pressure over a planar area to a continuously moving sheet form material passing betweeen a pair of press belts. The press includes a rigid frame with rollers spaced apart and rotatably supported in bearing supports forming a part of the press frame. The press belts include an upper and lower endless press belt each guided over a pair of rollers. Each press belt in combination with an associated pressure plate, located between the rollers, and a sliding surface seal located in a retaining member define a pressure chamber. The retaining member is positioned within a groove formed in the surface of the pressure plate facing the associated press belt and the retaining member is vertically displaceable relative to the press belt. A fluid pressure medium can be introduced into the pressure chamber for applying pressure to the associated press belt and, in turn, to the sheet form material passing between the press belts. The retaining member is secured by support members located within the pressure chamber.
Double band presses are used to press continuously moving sheet form materials, such as particle boards, impregnated paper, decorative laminates, impregnated glass fiber and natural fiber, woven webs, plastics material or rubber sheets. In the so-called isobaric machines of this type, a liquid or gaseous pressure medium is introduced into a pressure chamber defined between a pressure plate and the associated press belt and defined laterally by a sliding surface seal in contact with the moving press belts. As a result, the sheet form material passing between the two belts is compressed by the action of the pressure medium within the pressure chambers.
The arrangement of such sliding surface seals is disclosed in U.S. Pats. Nos. 4,193,342, 4,253,391, 4,331,073 and 4,285,525. The purpose of these various seal arrangements is to introduce the force resulting from the sliding friction between the seal and the surface of the press belt, chiefly from the surfaces of the seal extending parallel to the direction of movement of the press belt, into the pressure plate and then into the press frame whereby the seal, with the least possible cross-section and, accordingly, the highest possible flexibility, is capable of receiving the friction forces.
It is taught, particularly in U.S. Pat. No. 4,331,073, to avoid the risk of tilting the retaining member in the groove bounding the pressure chamber, which retaining member acts as a holder or mount for the seal, by a particular arrangement of the elastic seal and by dimensioning the groove and the retaining member in accordance with the desired object.
A disadvantage of the sliding surface seal disclosed in U.S. Pat. No. 4,331,073 is that the retaining member illustrated in FIGS. 2 and 3 can bear against the outer side of the groove when the inner pressure side is under the pressure of the pressure medium in the pressure chamber, wherein the groove seal arranged against the retaining member and in contact with the surface of the groove on the side open to the ambient atmosphere is crushed in the gap between the retaining member and the groove surface on the ambient atmosphere side.
When such a condition exists, the contact joint between the retaining member and the support member opens so that the function of the support member, that is, to prevent tilting of the retainting member, is no longer present. As a result, not only is there undesirable tilting of the sliding surface seal and the retaining member mounting it in the groove, but also an even more dangerous addition of the friction forces between the moving press belt and the sliding surface seal.
Since such sliding surface seals have a considerable expansion in the direction of movement of the press belts, the addition of the friction forces between the press belt and the sliding surface seal results in forces which tend to break the retaining member and result in a leakage of the pressure medium from the pressure chamber at the location where the break or rupture in the retaining member takes place. As a result, the entire pressure system becomes ineffective.